In recent years, as a mechano-electrical transducer for application to a driving element, a sensor, or the like, a piezoelectric material such as Pb(Zr, Ti)O3 has been used. Such a piezoelectric material in the form of a thin film formed over a substrate of Si or the like is expected to be applied to MEMS (micro-electro-mechanical systems) elements.
In producing a MEMS element, high-precision processing using a semiconductor process technology such as photolithography can be used, thus allowing the element to be reduced in size and to have an increased packing density. Particularly by densely and collectively forming elements on a relatively large Si wafer such as of 6 inches or 8 inches in diameter, compared with a case of single wafer production in which elements are produced one by one, significant cost reduction can be achieved.
Furthermore, with a piezoelectric material used in the form of a thin film and a device formed in a MEMS configuration, mechano-electrical transduction efficiency is improved, and this has led further to creation of new added values such as improvements in sensitivity and characteristics of the device. For example, in a case of a thermal sensor, having a MEMS configuration, the thermal sensor is reduced in thermal conductance, so that a measurement sensitivity thereof can be increased, and in a case of an ink-jet head for a printer, nozzles thereof are provided at an increased packing density, so that high-definition patterning can be performed.
By the way, a thin film of a piezoelectric material (hereinafter, referred to also as a piezoelectric thin film) formed over an Si substrate has, due to a difference in crystal lattice constant from that of Si, a polycrystal (columnar crystal) structure in which a plurality of crystals gather together in the form of an assembly of columns. It is known that the greater the amount of such columnar crystals grown on a common crystal plane in the film thickness direction (the higher an orientation characteristic), and the larger the columnar crystals, the higher piezoelectric characteristics of the film.
A device using a piezoelectric thin film adopts a configuration in which, on a substrate of Si or the like, a lower electrode, a piezoelectric thin film, and an upper electrode are layered in this order. In this configuration, each layer is formed by using a layer underlying it as its base and thus is grown while being affected to no small degree by the underlying layer. That is, taking note of the piezoelectric thin film, the more excellent crystallinity of the lower electrode underlying it, the more excellent crystallinity of the piezoelectric thin film. Furthermore, the larger a grain size of a metal constituting the lower electrode, the larger columnar crystals of the piezoelectric thin film, and the higher an orientation characteristic of the piezoelectric thin film. Moreover, a surface roughness of the lower electrode affects crystal growth at an early stage of formation of the piezoelectric thin film and thus is a key factor that affects the orientation characteristic of the piezoelectric thin film.
From this viewpoint, for example, in Patent Document 1, on a substrate, a seed layer containing at least one element constituting a piezoelectric thin film and a lower electrode are layered in this order, and the above-described element contained in the seed layer is diffused so that the surface of the lower electrode has an arithmetic mean roughness Ra of 0.5 to 30.0 nm, in an attempt to improve a crystal orientation characteristic of the piezoelectric thin film on the lower electrode. The technical idea behind this is that, conceivably, the diffusion of the above-described element causes a precipitate to be formed on the surface of the lower electrode, and the piezoelectric thin film is grown from the precipitate as a nucleus.
Furthermore, in Patent Document 2, in forming an electrode film on a substrate, first, initial crystal nuclei of an electrode material are formed in a pattern of islands on the substrate (Step A), and then, the above-described initial crystal nuclei are grown to form a growth layer of the electrode material (Step B). At this time, a substrate temperature in Step A is set to be higher than that in Step B, so that an electrode film having excellent crystallinity is formed. Patent Document 2 discloses that a full width at half maximum of a rocking curve of the electrode film is 1.80°, and this attests to the fact that the electrode film thus formed has excellent crystallinity.
Furthermore, in Patent Document 3, in a dielectric thin film capacitor composed of a dielectric thin film having a perovskite-type crystal structure interposed between upper and lower electrodes, a Pt layer as the lower electrode is set to have an average crystal grain size of not more than 50 nm, and a full width at half maximum of a rocking curve in X-ray diffraction of a (111) plane of Pt is set to not more than 5°, so that crystallinity and roughness of the dielectric thin film overlying the Pt layer are ameliorated to improve characteristics (for example, a relative dielectric constant) of the dielectric thin film.
Furthermore, in Patent Document 4, in a piezoelectric element in which a Pt layer as a lower electrode is formed over a substrate, and a PZT thin film is formed on the Pt layer, a full width at half maximum of a rocking curve of the Pt layer is set to not more than 5°, so that a crystal orientation characteristic of the PZT thin film overlying the Pt layer is improved to provide a large piezoelectric displacement amount.